1. Field of the Invention
The present invention relates to beam aberration correction, and more specifically, to the correction of phase and birefringence aberrations with seeded Brillouin scattering and Brillouin enhanced four wave mixing.
2. Description of Related Art
In the attempt to achieve greater power and energy capability from a laser system, it is necessary to increase the diameter of the optics used therein. As the size of optics and amplification mediums increase, the thermal, material and birefringence distortions increase beyond that which can be accepted in a usable system. Two main methods have been used to address this problem thus far: slab amplifier design and phase conjugation. The slab design is expensive to incorporate and does not completely correct for the previous problems mentioned.
U.S. Pat. No. 4,778,261, directed to a method and apparatus for optical phase conjugation, which apparatus is also known as a phase conjugate mirror, utilizing Brillouin enhanced four wave mixing in which a forward going pump wave and a backward going pump wave have frequencies which differ by twice the Brillouin frequency of the medium. The probe wave and the conjugate wave, which is obtained by Brillouin enhanced four wave mixing in the medium, have the same frequency, which differs from the pump wave frequencies by the Brillouin frequency of the four wave mixing medium. High reflectivity (gain) is obtained as a result of Brillouin resonance enhancement. The backward going pump wave is created from the forward going pump wave which is transmitted through the four wave mixing medium. The two pump waves are therefore phase conjugates of each other and the quality of the phase conjugation process is not degraded even by the use of an aberrated pump wave.
U.S. Pat. No. 4,869,578, directed to a gasdynamic gas flow, forms a lightguide wherein an incident coherent light wavefront, such as a laser beam, is reflected to produce a backward-traveling phase conjugated light wavefront. The nonlinear optical phase conjugation process occurs when the incident light intensity is large enough to achieve the intensity threshold required to initiate the phase conjugation process.
U.S. Pat. No. 4,958,908 is directed to a coherent input beam from a laser which is fed into a Brillouin-enhanced four wave mixer which generates a phase conjugated seed beam in counterpropagation with the input beam. A Brillouin amplifier is provided between the laser and four wave mixer to transfer energy from the input beam to the seed beam and thereby amplify the seed beam to produce a phase conjugated output beam with approximately 50% of the energy of the input beam and narrow bandwidth which is free from random phase jumps associated with the acoustic noise generally required to initiate and sustain stimulated Brillouin scattering. The phase conjugated output beam can be modulated or steered in any direction as desired. Reference beams for the four wave mixer may be derived from the same laser source as the input beam and have energies one or more orders of magnitude smaller than the energy of the input beam.
U.S. Pat. No. 5,155,542, discloses a coherent energy transfer system which utilizes a doublet pulse laser transmitter in combination with a phase conjugate mirror (PCM) assembly. In one embodiment of the present invention, the first pulse of the doublet pulse signal illuminates the target, whereas the return pulse reflected from the target constitutes a weak signal to be processed using phase conjugate methods. The second pulse of the doublet phase signal activates the PCM assembly and constitutes the energy source used to produce an amplified, phase conjugated beam.